A thin film corresponds to a layer of material deposited on a solid support or substrate, wherein the layer ranges in thickness from fractions of a nanometer (monolayer) to several micrometers. Thin films are employed, for example, in electronics (e.g., insulators, semiconductors, or conductors for integrated circuits), optical coatings (e.g., reflective, anti-reflective coatings, or self-cleaning glass) and packaging (e.g., aluminum-coated PET film).
Thin film deposition may be accomplished using a variety of gas phase chemical and/or physical vapor deposition techniques. Many of these deposition techniques are able to control layer thickness within a few tens of nanometers. Thin film deposition is also achieved by liquid phase and electrochemical techniques where the thickness of the final film is not well controlled. Examples include copper deposition by electroplating and sol gel deposition.
Gas phase deposition techniques fall into two broad categories, depending on whether the process is primarily chemical or physical. In a chemical deposition process, a precursor undergoes a chemical change at a solid surface, leaving a solid layer on the surface. In a chemical vapor deposition (CVD) process, a gas-phase precursor, often a halide or hydride of the element to be deposited, reacts with a substrate on the surface, leading to formation of the thin film on the surface.
The molecular precursors used in CVD are composed of at least one central element and at least one ligand bound to the central element. Upon reaction of the molecular precursor with the substrate, the central element is incorporated in the thin film. The ligand usually leaves the system as a gas phase reaction product.
One example of a unimolecular CVD reaction is the reaction of silane (SiH4) to form silicon thin films. The reaction is SiH4→Si+2H2. A silicon atom is the central element in the SiH4 molecule, and hydrogen atoms are ligands bound to the central element. H2 is the gas phase reaction product.
One example of a bimolecular CVD reaction is the growth of TiN using TiCl4 and NH3. This reaction is 6 TiCl4+8 NH3→6TiN+N2+24 HCl. In this bimolecular reaction, Ti is the central element in the TiCl4 molecule, and Cl atoms are ligands bound to the central element Ti. N is the central element in the NH3 molecule, and H atoms are ligands bound to the central element N. HCl is the gas phase reaction product.
CVD reactions are usually run at temperatures that allow for desorption of the reaction products from the surface of the growing film. In the case of silicon deposition using SiH4, the thin film growth is kinetically limited by H2 desorption from the silicon surface at the lowest possible temperatures for silicon thin film growth. In the case of TiN deposition using TiCl4 and NH3, TiN growth is limited by HCl desorption at the lowest possible temperatures for TiN thin film growth.
Such desorption temperature may be rather high in practice. For example, for silicon growth using SiH4, the H2 desorption temperature occurs at about 500° C., and thus silicon growth requires temperatures higher than 500° C. (generally 900-1100° C.). Similarly, the minimum temperature for TiN growth using TiCl4 and NH3 is greater than about 500° C., presumably because effective HCl desorption requires such temperatures. At temperatures lower than the desorption temperature for the HCl reaction product, the H and Cl ligands remain bound to the TiN surface, thus blocking sites needed for additional thin film growth. The CVD growth temperatures for diamond are generally 800-1100° C., for SiC are generally 1400-1550° C., and for GaN are generally 800-1100° C.
Electron stimulated desorption (ESD) is a technique used to remove species from surfaces at low temperature. ESD has been used in a method known as electron stimulated desorption in ion angular distributions (ESDIAD) to perform surface analysis.
There is a need in the art for novel methods of promoting thin film growth at low temperatures. In one aspect, such methods should allow for the atom-level control of the film growth, while using lower and more manageable experimental temperatures. In another aspect, such methods should allow for film growth on a macroscopic scale. The present invention meets this need.